This invention relates to suspension systems and in particular to a tandem axle suspension.
Conventional tandem axle assemblies for trucks or trailers are constructed with four leaf springs, two on each side of the frame. The remote ends of each pair of springs are usually supported in sliding contact with a fixed bearing surface, within suitable hanger brackets which are secured to the frame. The adjacent ends of each pair of springs are supported in sliding contact with fixed bearing surfaces within a pivotally mounted equalizer of one type or another. The equalizers are in turn supported by suitable hanger brackets which are secured to the frame.
Thus, the consequence of the arrangement for the equalized suspension described above is that if the front wheels in the tandem suspension, for example, should encounter a condition in the road producing a severe vertical movement of the front axle, the resultant deflection in the front springs will be transmitted in part by the equalizer to the rear springs. This results in an equalizing effect of the applied loads on the roadway for a tandem axle vehicle in which any abnormal vertical displacement of either axle, in the negative or positive sense, is distributed between the springs so as to minimize the weight differential between the axles. It can be appreciated, therefore, that the greater the equalizing capability of a tandem axle suspension, the less the chance of damage to the lading, roadway, pavement or bridge structure due to concentrated loading of either axle.
In the instance of a vehicle equipped with a prior art equalization tandem of the general type described above one problem that has been encountered, and is common to tandem axle suspensions with similar equalizing mechanisms, is that of inadequate load equalization from one axle to another. This condition occurs primarily as a result of the development of greatly unequal moments about the center pivot points of the equalizing mechanism when in operation. More specifically this is due to the attitude of the integral or fixed equalizer bearing pads relative to the spring ends as equalizer rotation occurs. As one end of the equalizer rotates upwardly the point at which the spring end contacts the integral bearing pad moves in the inboard direction. Concurrently the opposite end of the equalizer moves downwardly and the point at which its respective spring end contacts its integral bearing pad moves in the outboard direction. This action causes unequal moments about the center pivot point of the equalizer; inequality increases as the arc of equalizer movement increases.
Another contributing factor to the problem of inadequate load distribution is friction within the suspension. One major source of this friction is the contact of the outer or remote spring ends with the fixed bearing surfaces. As the spring is loaded the elongation or extension laterally due to the flattening of the cambered leaves necessitates spring movement relative to the bearing pad. In order for this relative movement (and ultimately equalization) to occur a force must be introduced of a magnitude sufficient to overcome static friction and allow this relative movement. If, for example, the wheels of an axle encounter a bump which produces a vertical force below the minimum necessary to overcome the static or break-away friction within that particular unit no relative movement occurs between the spring ends and the known bearing pads and, thusly, no load equalization.
Working concurrently these two factors cause inadequate load distribution throughout the full range of bumps and chuck holes which a tandem suspension might encounter in service. As bumps of small magnitude are encountered such as concrete highway expansion joints, the resultant force is often inadequate to overcome friction and momentary unequal load distribution occurs. As bumps of greater magnitude are encountered and resultant forces are adequate to overcome friction the unequal moments about the center pivot points of the equalizing mechanisms come into play, hindering proper load equalization.
The end results of improper load equalization, whether momentary or sustained, are many fold. If the axle which encounters bumps and overloads is of the unpowered type a resultant loss of traction occurs on the powered vehicle. The suspension components, and springs in particular, are subjected to higher stresses and therefore their service life is shortened appreciably. As the springs are subjected to overload, being of the progressive deflection-rate type, they progressively lose capacity to absorb energy and therefore transmit more energy through the mounting brackets to the vehicle frame and adversely affect the ride quality of the vehicle. This increased input of energy into the vehicle frame can often contribute, at given loads, speeds, highway conditions, etc., to resonant harmonic conditions which manifest themselves as vibrations of varying frequencies and amplitudes. These conditions are injurious to the vehicle and indeed are a source of driver fatique, discomfort and injury. A final consideration is that, wherever an unequal load distribution occurs within a tandem suspension, the axle which is overloaded transmits its load to the roadway which is detrimental to the roadway, particularly in the case of shallow based asphalt roads or new unsettled highway pavement.
The suspension system of the present invention includes pivotally mounted shackles interposed between the adjacent ends of the leaf springs and the equalizer beam ends which support the adjacent ends of the leaf springs. As equalizer rotation occurs, although the spring ends and shackles change in attitude relative to the equalizer beams, the points at which they transmit their load to the equalizer beams remain substantially equal inasmuch as they are defined by the arcuate path on which they travel about the center pivot of the equalizer mechanism during equalizer rotation. Consequently, the moments about the center pivot points of the equalizer mechanism remain much more nearly equal, resulting in more even distribution of load from one axle of the tandem to the other. Further, the remote end of each of the four springs is mounted upon and supported by a low friction roller which virtually eliminates the friction associated with the prior practice of establishing sliding contact between the spring ends and hanger bearing surfaces. With the use of pivotally mounted shackles on the adjacent spring ends and low-friction rollers on the remote spring ends friction is greatly reduced allowing the equalizing mechanism of the tandem to function properly upon the introduction of correspondingly reduced force magnitudes. Further, the use of pivotally mounted shackles on the adjacent spring ends and low-friction rollers on the remote spring ends eliminates the wear to both springs and bearing surfaces of the type normally associated with suspensions of the prior art.